The Awesomeness of Beecher's Trilobite Bed

Trilobites are remarkable animals, deserving not only multiple posts, but probably their own blog (if one exists, I haven’t found it yet…). They are one of the most abundant, well-known, and best-studied organisms from the fossil record. If anyone goes through a “rockhound” period in his or her life, it’s almost a guarantee they picked up a fossil trilobite. For those unfamiliar, trilobites kind of looked like a cross between a pill bug and cockroach (see below), but lived in the ocean. However, 99% of the time, it’s just the exoskeleton of trilobites that is preserved. For better preservation, there are two prime trilobite fossil beds in the world, the Hunsrück Shale of Germany, and Beecher’s Trilobite Bed in New York.

Beecher’s Trilobite Bed is the result of a turbidity flow (think an underwater “mudslide” of very muddy water, image at left from here) which picked up trilobites, carried them a short distance, and essentially buried them alive. For taphonomic reasons, this rapid burial exquisitely fossilized the trilobites as pyrite, fossilizing not only exoskeletons but legs, antennae, muscles, and even internal organs (image below from PDF here). For a more lengthy explanation of how Beecher’s Trilobite Bed was formed, there’s an excellent chapter by Etter (2002), and Cisne’s original taphonomic study from 1973 is now publicly available. But the Beecher bed is a definite taphonomic outlier for a couple reasons, both of which probably need a brief introduction:     

Trilobites were rebels before it was cool

First, in the ocean, only three things are needed to produce pyrite (iron sulfide: FeS₂): sulfur, iron, and organic carbon. Chemical studies of the Beecher bed (Briggs et al., 1991) suggest the seawater was already rich in sulfur and iron – all that was needed was organic carbon, readily provided by the washed-in trilobites. Due to the abundance of sulfur and iron, it is plausible that every trilobite (as in, 100%) washed in by the turbidity flow acted as a nucleation site for pyrite and ended up getting fossilized. This is highly unusual – preserving 10% of a paleo-environment would make a paleontologist drool (personal observation).

Second, pyrite can form two ways in the ocean: through “bacterial sulfate reduction” (BSR) or through diagenesis (after burial). BSR occurs in open seawater, while diagenesis occurs in the seabed sediment, with no contact with the seawater above. How the pyrite is derived affects the sulfur isotopes in the mineral: it is commonly thought that BSR-derived pyrite is enriched in “lighter” ³²S, while diagenetic pyrite is enriched in “heavier” ³⁴S. Understandably, soft parts (like legs and antennae) decompose faster than hard parts (like exoskeletons), so soft parts would need to fossilize first, right? Well, as it turns out, the legs and antennae of the Beecher trilobites tend to be enriched in the “heavier” ³⁴S, while the exoskeletons are enriched in the “lighter” ³²S (Briggs et al., 1991). This suggests the exoskeletons were fossilized first, before burial, and the softer parts stuck around and were fossilized after burial. This is kind of like a root beer float where the root beer evaporates before the ice cream even melts. Briggs et al hypothesized this could have occurred due to changes in water chemistry as the exoskeleton decomposed, but there could be a greater disturbance in the force...

Wait, what do you mean “it is commonly thought”?   

The paradigm on sulfur isotopes, as it turns out, was recently challenged by a publication in Geology led by Justin Ries, a marine geologist at The University of North Carolina (pictured in the field at left, from here). Ries and colleagues examined the sulfur isotopes in a 10 million-year stretch of carbonates from Namibia (Ries, 2009: PDF available at link above). They found that the pyrite derived by BSR was actually enriched in the heavier ³⁴S isotope, enough to be labeled “superheavy pyrite.” Needless to say, this throws a monkey wrench into the current notion that the bacterially-derived pyrite should be “lighter.”

The translation of this article towards the Beecher bed is uncertain – Ries et al attribute the anomalous pyrite isotopes to large-scale (possibly global) low atmospheric oxygen, but it is unlikely such conditions existed when the Beecher bed was deposited. They also consider the anomalous pyrite could have resulted from stratification of the water column, low water levels, or aerobic reoxidation (mixing oxygen back into the water). Personally, I could easily be convinced that turbidity flows, like the kind that produced the Beecher bed, could carry oxygenated water. But, to my knowledge, it’s uncertain how the isotopes of subsequent  diagenetic pyrite would be effected – i.e., could it end up being “lighter” than the bacterially-derived pyrite, or would it be “super-duper heavy”? Like any good scientific study, it creates more questions than it answers… 

 

REFS

Briggs, D.E.G., Bottrell, S.H., and Raiswell, R., 1991, Pyritization of soft-bodied fossils: Beecher’s Trilobite Bed, Upper Ordovician, New York State, Geology, v. 19, p. 1221–1224. 

Cisne, J.L., 1973, Beecher’s Trilobite Bed revisited: ecology of an Ordovician deepwater fauna, Postilla, v. 160, p. 1–25.

Etter, W., 2002, Beecher’s Trilobite Bed: Ordovician pyritization for the other half of the trilobite, in Bottjer, D.J., Etter, W., Hagadorn, J.W., and Tang, C.M., eds, Exceptional Fossil Preservation: a Unique View on the Evolution of Marine Life, Columbia University Press, New York, p. 131–142.

Ries, J.B., Fike, D.A., Pratt, L.M., Lyons, T.W., and Grotzinger, J.P., 2009, Superheavy pyrite (δ³⁴S_pyr > δ³⁴S_CAS) in the terminal Proterozoic Nama Group, southern Namibia: a consequence of low seawater sulfate at the dawn of animal life, Geology, v. 37, p. 743–746.